LEDs - condensed matter/nanostructures having real impact

I'd written two years ago about the pervasiveness of light emitting diodes for holiday decorations.  While revising some notes for my class on nanoscience and nanotechnology, I recently came upon some numbers that really highlight the LED as a great example of condensed matter (and recently nanoscience) having a serious positive impact on energy consumption and environmental impacts. 

Image from here.

Back when I was growing up, incandescent light bulbs were common, and pretty lousy at generating light for a given amount of energy input.  Incandescents produce something like 20 lumens/W, while compact fluorescent bulbs are more like 60 lm/W.  In contrast, LED lighting is well over 100 lm/W and is hitting numbers like 200 lm/W in more expensive bulbs, and in theory could reach more like 325 lm/W.   (For good sources of information about this, I recommend this report by the International Energy Agency, and this 2020 report (pdf) from the US Department of Energy.   LED "white" lighting works either by having a UV LED that excites the same kind of phosphors that are in fluorescent bulbs, or by "color mixing" through having red, green, and blue LEDs all in one package.  (The "nano" comes into this both through the precision growth of the semiconductors and in some cases nanostructuring to enhance the fraction of emitted light that actually gets out of the LED.)

Six years ago, lighting accounted for about 15% of global electricity demand.  In just a few years, LEDs have gone from a few % of market share for new lighting to well above 50% of market share, and there is no sign of this slowing down.  The transition to LEDs is expected to save hundreds of billions of dollars per year in energy costs, gigatons per year in CO₂ emissions, and to stave off the need to construct over a hundred new municipal-scale power plants over the next decade.  

This is a big deal.  One way to cast "the energy problem" is that there is no clear, environmentally reasonable path toward raising the standard of living of billions of people up to the level of per capita energy consumption seen in the most developed economies.  Cutting that per capita energy use would be great, and LED lighting is a true success story in that regard.  

Hanle magnetoresistance - always more to learn....

You would think that, by now, we would have figured out basically all there is to know about comparatively simple metals conduct electricity, even in the presence of a magnetic field.  I mean, Maxwell and Faraday etc. were figuring out electric and magnetic fields a century and a half ago.  Lorentz wrote down the force on a moving charge in a magnetic field in 1895.  The Hall Effect goes back to 1879.  Sommerfeld and his intellectual progeny laid the groundwork for a quantum theory of electronic conduction starting about a hundred years ago.  We have had good techniques for measuring electrical resistances (that is, sourcing a current and measuring the voltage differences between different places on a material) for many decades, and high quality magnets for around as long.  

Surprisingly, even in very recent times we are still finding out previously unknown effects that influence the resistance of a metal in a magnetic field.  Let me give you an example.  

I'd written here about the spin Hall effect and its inverse, which were only "discovered" relatively recently.  In brief, because of strong spin-orbit coupling (SOC) effects on the electronic structure of comparatively heavy metals (Pt, Ta, W), passing a current through a thin film strip of such a material generates a spin current, leading to the accumulation of spin at the top and bottom of the strip.  If those interfaces are in contact with magnetic materials, exchange processes can take place so that there is a net transfer of angular momentum between the metal and the magnetic system.  

There is actually a correction to the resistance of the SOC metal:  The spin accumulation can lead to a diffusive spin current between the top and bottom surfaces, which (thanks to the inverse spin Hall effect, ISHE) gives an additive kick to the charge current (and effectively lowers the resistance of the metal from what it would be in the absence of the spin Hall physics).  If the top and bottom interfaces are in contact with a magnetic system and therefore affect the spin accumulation, that correction can be modified depending on the orientation of the magnetization of the magnetic material, leading to the spin Hall magnetoresistance.  

Spin Hall/inverse spin Hall
resistive correction,
adapted from here.

That's not the end of the story, however.  Even without an adjoining magnetic material, there is an additional magnetoresistive correction, \(\delta \rho(\mathbf{H})\) to the resistivity of the SOC metal.  If the magnetic field has a component transverse to the direction of the SHE accumulated spins, the spins will precess about that field, and that can affect the ISH correction to the resistivity.  This was predicted in 2007 by Dyakanov (arxiv, PRL), and it was found experimentally several years later, as reported in PRL (arxiv version here).  There are readily measurable effects in both the longitudinal resistivity \(\rho_{xx}\) (voltage measured along the direction of the current) and the transverse resistivity \(\rho_{xy}\) (voltage measured transverse to the current, as in the Hall effect, but this holds even when the external magnetic field is in the plane of the film).

Hanle magnetoresistance idea, 
adapted from here.

This correction is called the Hanle magnetoresistance.  

(Aside:  There is some interesting scientific history behind the name.  Hanle was the first to explain an atomic physics optical effect, where the precession of magnetic moments of a gas of atoms in a magnetic field affects the polarization of light passing through the gas.  In condensed matter, the name "Hanle effect" shows up in discussions of spin transport in metals.  The first time I ever encountered the term was in this paper, which foreshadows the discovery of giant magnetoresistance.  A ferromagnetic emitter contact is used to inject spin-polarized electrons into a non-magnetic metal, aluminum.  Those electrons diffuse over to a second ferromagnetic collector contact, where their ability to enter that contact (and hence the resistance of the gadget) depends on the relative alignment of the spins and the magnetization of the collector.  If there is a magnetic field perpendicular to the plane of the device, the spins precess while the electrons diffuse, and one can analyze the magnetoresistance to infer the spin relaxation time in the metal.)

One of my students and I have been scratching our heads trying to see if we really understand the Hanle magnetoresistance, which we have been measuring recently as a by-product of other work.  I think it's pretty amazing that we are still discovering new effects in something as simple as the resistance of a metal in a magnetic field.

The community of department chairs

For the vast majority, there is no formal training process that professors go through before-hand to become chair or head of a department.  That makes access to the experiences and knowledge of others an invaluable resource.  In recent years, the APS has been sponsoring conferences of physics (or physics & astronomy) department chairs, and that's great, but pre-dating that have been electronic mailing lists for department chairs and heads*.  There is a long-standing, somewhat appropriately named "Midwest Physics Department Chairs" email listserv, and similarly there is an analogous American Astronomical Society astronomy chairs listserv.  

The chairs mailing list has been a great way to learn how processes work at other places, and to get advice or sanity checks.  Sometimes it can be very helpful to be able to say to your administration, "here is how everyone else does this."  Not everything translates, as large public universities have some real structural differences in operations compared to private universities, but it's still been informative. Examples of recent discussion topics in no particular order:

• Rough startup costs for hires in different subfields (and how those costs are borne between departments, deans, provosts, etc.)
• Qualifying/candidacy exams - what they cover (undergrad v grad), their value or lack thereof
• Promotion and tenure processes 
• Diversity/equity/inclusion at all levels
• Graduate admissions in the post-standardized-test era
• International students in the era of covid + recent changes in student visa policies
• Various curricular issues (incorporating computation; lab staffing)
• Mental health at all levels (undergrads, grad students, faculty, staff)

The group also has an annual get-together.  Last weekend I attended a meeting (face to face!) of about 30 physics department chairs at the exotic O'Hare Airport Hilton in Chicago.  While not everyone was able to make it, it was helpful to talk and compare notes.  People had a lot to say about teaching methods and what will stick around post-pandemic.  It was also very informative to learn what it takes financially and in terms of personnel to support a successful bridge program.  

Being chair or head can be isolating, and it's good having a community of people who understand the weird issues that can come up.  

* The definitions are not rigid, but a chair is often elected and expected to make decisions through consensus and voting, while a head is appointed and typically has more autonomy and authority.  As one former head at a big place once told me, though, you basically need consensus as a head, too, otherwise you can't get anything done.

The noise is the signal

 I am about to attend a gathering of some physics department chairs/heads from around the US, and I'll write some about that after the meeting, but I wanted to point out a really neat paper (arxiv version here) in a recent issue of Science.  A group at Leiden has outfitted their scanning tunneling microscope with the ability to measure not just the tunneling current, but the noise in the tunneling current, specifically the "shot noise" that results out of equilibrium because charge is transported by the tunneling of discrete carriers.  See here for a pretty extensive discussion about how charge shot noise is a way to determine experimentally whether electrons are tunneling one at a time independently, or whether they are, for example, being transported two at a time because of some kind of pairing.

Adapted from Fig. 1 of this paper.

The experiment is quite pretty, looking at disordered thin films of TiN, with a macroscopic superconducting transition temperature of \(T_{c} =\) 2.95 K.  With the shot noise measurement, the experimenters see enhanced noise at low applied voltages consistent with pairing (with pairs being transported presumably by the process of Andreev reflection).  The interesting point is that this enhanced noise persists up to temperatures as high as 2.7 times \(T_{c}\), despite the fact that the tunneling conductance \(dI/dV\) shows no sign of a gap or pseudogap up there.  This implies that superconductivity in this material dies as \(T\) exceeds \(T_{c}\) not because the pairing between electrons falls apart, but instead because of the loss of the global coherence needed for the superconducting state.  That's an exciting result.  

I'm a big fan of noise measurements and applying them to a broader class of condensed matter systems.  We'd seen enhanced noise in cuprate tunnel junctions above \(T_{c}\) and at large biases, as mentioned here, but in the cuprates such persistence of pairing is less surprising than in the comparatively "simple" TiN system.  Noise measurements on demand via STM should be quite the enabling capability!